Bacterial membrane-disintegrating peptides, which have their origin in naturally occurring cationic peptides, offer promising alternatives as antibiotics of the future. These novel agents generally demonstrate a broad spectrum of antibacterial activity and act by disrupting the integrity of the entire bacterial cell membrane, thereby reducing the risk of drug resistance (M. L. Cohen, Science 257, 1050-55 (1992); and A. M. D. Virk et al., Mayo Clinic Proc. 75, 200-214 (2000). Antibacterial peptides have two major distinguishing features: a net positive charge, typically of +2 to +6, and an overall amphipathic fold imparting polar and hydrophobic faces to the molecule (A. Giangaspero et al., Eur. J. Biochem. 268, 5589-5600 (2001). The cationic nature of antibacterial peptides apparently promotes selective interaction with the negatively charged surface of bacterial membranes relative to the more neutral surface of eukaryotic membranes (K. Matsuzaki et al., Biochemistry 36, 9799-9806 (1997). Once attracted to the surface, the peptide, with its amphipathic topology, triggers bacterial cell lysis (D. Andreu et al., Biochemistry 24, 1683-1688 (1985).
Sepsis and septic shock are systemic complications normally associated with increased levels of lipopolysaccharide (LPS) endotoxin in the blood stream. Many bactericidal peptides are known in vitro to bind to and to neutralize LPS (e.g., cecropins, magainins, proline-arginine-rich peptides, sapecin, tachyplesin, and defensins) (Andreu et al., Biopolymers, 47; 415-33 (1998)) as well as, more recently, βpep peptides, (Mayo et al., Protein Sci., 5; 13001-1315 (1996); Mayo et al., Biochem. Biophys. Acta, 1425; 81-92 (1998)). SC4 (Mayo et al., Biochem. J., 349(3); 717-28 (2000)), and lactoferrin-based peptide LF11 (Japelj et al., J. Biol. Chem., 280; 16955-61 (2005)). Perhaps the most prototypic is polymyxin B (PmxB), a small cyclic lipopeptide (Rifkind, J. Bacteriol., 93; 1463-4 (1967)). However, due to its high neuro- and nephrotoxicity, PmxB is limited to topical application, and most other bactericidal agents are not very effective against LPS in vivo.
Some naturally occurring bactericidal proteins also possess endotoxin-neutralizing properties. Several non-peptidic membrane disrupters have also been identified (Lockwood et al., Drugs of the Future 28, 911-923 (2003). Most prominent of these is squalamine, which is amphipathic not by the nature of its folded structure, but by the presence of charged appendages (including the polycationic triamine) on a steroid core (Moore et al. Proc Natl. Acad. Sci. USA 90, 1354-1358 (1993). The bactericidal mechanism of squalamine, despite its small size, is similar to that of membrane-disintegrating peptides (Selinsky et al., Biochim. Biophys. Acta 1370, 218-234 (1998); Selinsky et al., Biochim. Biophys. Acta 1464, 135-141 (2000).
A structural survey of these peptides reveals that regardless of their folded conformation, two traits stand out that are important for binding LPS: amphipathic character and a net positive charge (Lockwood et al., Drugs of the Future, 28; 911-923 (2003)). Presumably, positively charged residues from the peptide promote interaction with negatively charged groups on LPS, i.e., phosphates on the lipid A glucosamines and/or those in the inner core polysaccharide unit, while hydrophobic residues from the peptide interact with acyl chains on lipid A. Structural studies of peptides in complex with LPS support this notion and have provided additional insight into the molecular origins of peptide-mediated LPS neutralization (Japelj et al., J. Biol. Chem., 280; 16955-61 (2005); Ferguson et al., Science, 282; 2215-20 (1998); Pristovsek et al., J. Med. Chem., 42; 4604-13 (1999)).
Interestingly, the motif of a positively charged, amphipathic structure (primarily anti-parallel β-sheet) is also found in a number of proteins and peptides that function as antiangiogenic agents (Dings et al., Angiogenesis, 6; 83-91 (2003)). For example, angiostatin folds into an anti-parallel β-sheet structure with a highly electropositive lysine-rich binding site (Abad, J. Mol. Biol. 318; 1009-17 (2002)). Endostatin has a predominantly anti-parallel β-sheet structure (Hohenester et al., EMBO J., 17; 1656-1664 (1998)) and is highly positively charged, particularly due to the presence of multiple arginine residues. Angiogenesis, the process of new blood vessel formation, is key to normal organ development, as well as to various pathological disorders like cancer, arthritis, endometriosis, diabetic retinopathy, and restenosis (Griffioen et al., Pharmacol. Rev., 52; 237-68 (2000)). The use of agents that can inhibit angiogenesis, particularly in anti-tumor research, has indicated that anti-angiogenic therapy is a promising therapeutic modality (Boehm et al., Nature, 390; 404-7 (1997).
In the last decade or so, researchers have begun to develop modified or totally synthetic peptides (Sitaram et al., Int. J. Pept. Protein Res. 46, 166-173 (1995); Saberwal et al., Biochim. Biophys. Acta 984, 360-364 (1989); Tossi et al., Eur. J. Biochem. 250, 549-558 (1997); Blondelle et al., Antimicrob. Agents Chemother. 40, 1067-1071 (1996); Dathe et al., Biochim. Biophys. Acta 1462, 71-87 (1999); and Beven et al., Eur. J. Biochem. 270, 2207-2217 (2003)). Some resultant amphipathic peptides show promising broad bactericidal activity and specificity for bacterial rather than eukaryotic cells (R. E. Hancock, Lancet 349, 418-422 (1997); Hancock et al., Adv. Microb. Physiol. 37, 135-175 (1995). Little has been done to design non-peptide topomimetic compounds that mimic a portion of the surface of a protein or peptide. Additional topomimetic compounds are still needed.